- Define Unit type as int for type-safe time unit constants
- Implement all 8 unit constants (Seconds through Years) using iota
- Add String() method for debugging and error messages
- Comprehensive unit tests verifying constants, ordering, and comparability
- Type safety tests demonstrating compile-time safety
- Example tests showing usage patterns
- 100% test coverage (exceeds 95% requirement)
All acceptance criteria met:
✓ Unit type defined as int
✓ All 8 unit constants defined (Seconds through Years)
✓ Units use iota for ordering
✓ Godoc comments for Unit type and constants
✓ Unit tests verifying constants
